Field of the Invention
The present invention concerns a mechanical superconducting switch, in particular for a superconducting magnetic resonance imaging (MRI) magnet.
Description of the Prior Art
Superconducting MRI magnets are formed of several coils of superconducting wire electrically connected in series and conventionally housed within a cryostat with a cryogenic refrigerator which cools the magnet to below a superconducting transition temperature of the material of the coils.
Many conventional designs include a bath of liquid cryogen, for example liquid helium, which is maintained below its boiling point by a cryogenic refrigerator.
However, more recent designs have sought to reduce or eliminate consumption of cryogens such as helium, for example by using cooling loops or “dry” also referred to as cryogen free magnets in which no liquid cryogen is used.
It is necessary to provide a switch across the terminals of the series connection of coils. In one state (the “on” state), the switch should be superconducting, so as to complete a superconducting circuit through the coils so that current may flow persistently in the magnet. In another state (the “off” state), the switch should be resistive, to allow current to be introduced into, or removed from, the coils by a power supply unit connected to the magnet for the purpose. Conventionally, all superconducting switches require the fabrication of an ancillary superconducting coil used to effect the switching operation. The ancillary coil is typically formed of wire having a matrix typically made of a resistive CuNi alloy. This renders the switch susceptible to temperature and wire instabilities. The wire and filament size play an important role in the stability of the switch against flux jumping, in which a small quench in a single filament may propagate to the other filaments in the wire due to resistive dissipation in the matrix material carrying current between filaments.
Conventional superconducting switches have a limited open-circuit resistance and thus limit the achievable ramp rate and dissipate heat during energization and de-energization of the magnet. The conventional switches are opened and closed using a thermal heater in thermal contact with the ancillary superconducting coil. This heater contributes to heat load on the cryostat and heat dissipation. For example, a certain conventional design includes an ancillary coil with an “off” resistance of about 5-50Ω. This dissipates power during ramp up and during ramp down. The heaters themselves on the switch dissipate further energy during a ramp up or down. Such levels of heating are far in excess of the cooling power of a typical 4.2K cryogenic refrigerator. In cryostats with baths of liquid cryogen, the required cooling was provided by immersing the switch in the liquid bath.
The drive for dry magnets calls for a different approach to the superconducting switch as the cooling power at 4.2K is very limited, typically 1.2 W.
The following documents contain technical information relating to the background of the present invention:
Makoto Takayasu, Electric Characteristics of Contact Junctions Between NbTi Multifilamentary Wires, IEEE Transactions on Applied Superconductivity, Vol. 9, No. 3, September 1999.
Makoto Takayasu, Toshiaki Matsui, and Joseph V. Minervini, Negative-Resistance Voltage-Current Characteristics of Superconductor Contact Junctions for Macro-Scale Applications, IEEE Transactions on Applied Superconductivity, Vol. 13, No. 2, June 2003.
S. Ohtsuka, H. Ohtsubo, T. Nakamura, J. Suehiro, and M. Hara, Characteristics of NbTi mechanical persistent current switch and mechanism of superconducting connection at contact, Cryogenics 38 (1998) 1441-1444.
US2002/0190824 A1, dated Dec. 19, 2002: Persistent Current switch and method for the same.
JP7231125-A, CHODENDO MAGNET KK (CHOD-C); FURUKAWA ELECTRIC CO LTD (FURU-S) 1995-08-29, Persistent current switch examination method e.g. for magnetic-levitation train—involves letting circumference current and DC current flow along same direction to persistent current switch by second power supply lifted to both ends.
JP6350148-A, 1994-12-22, HITACHI LTD (HITA-S) Persistent current superconductive device for energy storage—incorporates superconducting wire, current lead and permanent current mechanical switch.
U.S. Pat. No. 5,532,638, dated Jul. 2, 1996, CHUBU DENRYOKU KK (CHUB-S); CHUBU ELECTRIC POWER CO (CHUB-S); HITACHI LTD (HITA-S), Superconductive energy storage device for the same.
E. M. Pavão, Critical Temperatures of Superconducting Solders. MIT. June 2007
CN100595856C. Chinese Academy of Science.